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Thread: Sail math and placement ?

  1. #1
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    Default Sail math and placement ?

    Since I have one, Iím going to use my Bolger 59 sail on my Michalak seal cove skiff. Now I understand that the dagger or leeboard is placed in relation to the center of effort. However, where do I stick the sail in the first place. It looks good at the first thwart. That seems to be how many designers have done it.

  2. #2
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    Default Re: Sail math and placement ?

    In general, you want the sail's CE for a single-sailed boat somewhere maybe 35%-50% aft. Sometimes the biggest determining factor is what sort of other interior structure you have to build from or onto. Most single-sail dinghies will place the sail's center of effort (and consequently the mast step and/or partners) so that the sail's CE has a "lead" (is ahead of the Center of Lateral Resistance of the hull) by about 10% of the boat's waterline length. The actual CLR can be pretty tough to pinpoint though due to variations in hull shapes. Plus, the underwater shape of a dinghy can vary drastically due just to the position of the sailor(s) in the boat and the boat's fore and aft trim. This can be both a good thing and a bad thing, but it will be something you can probably use to your advantage with practice. If I had one to rig and wanted to give myself the best shot at a workable situation most of the time, I'd probably shoot for about 40% aft for the CE and take it out and try it.

  3. #3
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    Default Re: Sail math and placement ?

    if intending to use a leeboard you have to approach the solution by beginning w/th the leeboard position

    best/easiest to position leeboards @ the widest point of the hull keeping the alignment parallel w/ the centerline of the hull

    once that has been decided on then find the CE of the sail and locate it directly above the center of the lrrboard

    you are not going for LOOKS you are striving for BALANCE so the rudder is neutral(no weather helm and no lee helm)

    some will not agree and desire a bit of weather helm which is up to them

    IIRC there is a good discussion of this issue on the PUDDLE DUCK RACER website w/ color pics

    it is in the LEEBOARD, DAGGERBOARDS, CENTERBOARDS, ETC section in the SITE MAP

    https://pdracer.com

    just remember the PDR's are rectangular so their widest beam is the whole side and your Michalak Seal Cove Skiff's maximum beam is somewhere about amidship longitudinallyĪ

    Ņ have i confused you enuff ?

    BON CHANCE

    sw
    "we are the people, our parents warned us about" (jb)

    steve

  4. #4
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    Default Re: Sail math and placement ?

    That’s not too confusing. I was planning on using the leeboard from the Bolger design and sticking it in a box to make a daggerboard. With CE at 40% aft that would put the dagger right forward of the middle thwart which is not a problem for space.

  5. #5
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    Default Re: Sail math and placement ?

    Keep in mind that your CE/CLR relationship is a guideline that you'll be constantly balancing by shifting your weight fore and aft, changing both.

  6. #6
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    Default Re: Sail math and placement ?

    That’s not too confusing. I was planning on using the leeboard from the Bolger design and sticking it in a box to make a daggerboard. With CE at 40% aft that would put the dagger right forward of the middle thwart which is not a problem for space.

  7. #7
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    Default Re: Sail math and placement ?

    I've had success changing rigs in a few boats: canoe, Folbot kayaks, Shellback dinghy. The rigs involved were lug, lateen, sprit and triangular sprit boom. The handling didn't vary enough to worry about. Use your best estimate. Good luck, keep us posted.

  8. #8
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    Default Re: Sail math and placement ?

    Getting close to sailing in this one.

  9. #9
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    Default Re: Sail math and placement ?

    Going back a bit, the quick method to find out your center of lateral resistance (without math!) is to cut out a cardboard profile of the underwater body, including the dagger/bilge board and balance it on a ruler. that gives the vertical line of the CLR. Put your centre of sail area 8 to 12% ahead of that and you have a good start point.
    As above, your, and other bodies will alter things considerably in a small boat.

  10. #10
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    Default Re: Sail math and placement ?

    I am not familiar with the boat you are dealing with but this balance thing can be a black art: No less than Howard Chapelle got it wrong on the Western Lakes Mackinaw shown in his book American Small Sailing Craft. I am aware of two boats that were built to his drawing and both had too much weather helm. One cut down the mizzen to the first reef point to compensate. So I built a 1:8 scale model from the lines with radio control and it also had a weather helm. I messed about with it, increasing the main and shortening the mizzen until the "lead" (distance of the center of effort forward of the center of lateral resistance) was about 10% of the LWL instead of the 3% that Chapelle drew. The model and now the full-sized boat both balance well, though I think I will still work on trimming the big boat more down by the stern so I can carry more mizzen when the wind pipes up. I have read that full-bowed boats often need more lead than more slender ones, and this fits the case of the Mackinaw. Having said all that, can you mock up something that allows you to move the stuff around and get empirical on it? It could be fun...

    Ken

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